Spatial Light Modulators are widely used in xerographic printers, video monitors, and projection TV's. These SLM's are typically comprised of a linear or area array of pixels. The pixels modulate incident light to form an optical image which exposes a photoreceptor drum in the case of a printer, and which modulated light illuminates a projection screen in the case of a video monitor or backscreen projector.
One type of micro-mechanical spatial light modulator is the digital micromirror device (DMD), also known as a deformable mirror device, manufactured by Texas Instruments Incorporated of Dallas Tex. The DMD is a single monolithic integrated circuit including an area or linear array of hundreds or thousands of microsized bistable tilting mirrors. Light incident on each mirror of the DMD pixel mirror array is selectively reflected either towards or away from an image plane to form light images. Each mirror is pivotably attached to at least one support post via one or more hinges. The mirror is spaced above underlying control circuitry comprising addressing electrodes, these addressing electrodes providing electrostatic forces to cause the mirror to selectively tilt downward in one direction or the other, reflecting incident light one of two directions.
For more detailed background of the implementation of a DMD in a video display or projection T.V., cross reference is made to U.S. Pat. No. 5,206,629 to DeMond, et al entitled "Spatial Light Modulator and Memory for Digitized Video Display", and U.S. Pat. No. 5,096,279 to Hornbeck, et al, entitled "Spatial Light Modulator and Method", both these patents being assigned to the same assignee as the present invention, and the teachings incorporated herein by reference. For additional discussion of implementing a DMD in an electrostatic printer, cross reference is made to U.S. Pat. No. 5,101,236, to Nelson. et al, entitled "Light Energy Control System and Method of Operation", this patent being assigned to the same assignee as the present invention, and the teachings incorporated herein by reference.
DMD's have various designs, each pixel of which include the fabrication of at least one support post, at least one hinge, a beam, and in some cases, an elevated reflective layer fabricated upon the beam. Fabrication of these various structural elements have been accomplished with various semiconductor processing approaches. These approaches require various steps including sequentially depositing material layers one at a time, and patterning these layers using various masking and etching techniques. To establish these patterns, masking layers, such as sacrificial oxide masks, are sometimes used. The use of particular masks have various advantages and disadvantages, and contribute to the overall device complexity, uniformity, and yield level of the DMD device during fabrication. An example of one such fabrication process implementing a sacrificial oxide mask is disclosed in the cross referenced pending patent application entitled "Improved Hinge for Micromechanical Device", Ser. No. 08/268,741, filed Jun. 30, 1994, the teachings of which are included herein by reference.
The use of a sacrificial oxide hard mask to transfer a pattern to metal layers is primarily used to fabricate the DMD. The sacrificial oxide hard mask is a robust process, and is well suited for etching metal films on organic substrates. However, the sacrificial oxide hard mask has associated problems as well, including poor conformal metal deposition control after mask formation resulting in structural weaknesses where the beam metal is deposited over the hinge oxide mask. In addition, the oxide mask is patterned using a positive photoresist, this photoresist being selectively removed after oxide etch through an ash process and leaving an etch residue. Finally, there is the requirement of an oxide strip to remove the sacrificial oxide mask when used to form a hinge and beam in the DMD devices. The hinge can be damaged during the oxide strip if selectivity to the hinge material is low.
In the prior art, the DMD superstructure, namely, the support post, hinges and beam are typically comprised of aluminum or an aluminum alloy. Aluminum is a material well suited for fabricating the DMD due to its amenability to processing, and its high optical reflectivity.
In advancing the state of the art for manufacturing DMD's, it is desired to provide the DMD with a mechanically stronger superstructure. Moreover, it is desired to provide a simpler process than that currently used to fabricate the aluminum and aluminum alloy DMD, which implements a sacrificial oxide hard mask. These and other goals are achieved by the present invention.